1. Field of the Invention
The present invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method, and a computer program for mutual communication between plural wireless stations such as wireless LANs (Local Area Networks). More specifically, the present invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method, and a computer program for implementing broadband wireless transmission under communication environment such as in the home.
In more detail, the present invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method, and a computer program for enhancing the transmission capacity by means of communication (MIMO communication) based on space division multiplexing using a pair of a transmitter having plural antennas and a receiver having plural antennas. More particularly, the present invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method, and a computer program for performing MIMO transmission using singular value decomposition (SVD) of a channel information matrix composed of channels corresponding to antenna pairs for transmission and reception.
2. Description of Related Art
Computer networking such as LAN can be used to efficiently share information resources and hardware resources. Particular attention is paid to wireless LAN as a system to free users from LAN wiring based on conventional wired systems. The wireless LAN can eliminate most of cables in work places such as offices, making it possible to relatively easily move communication terminals such as personal computers (PCs).
In recent years, there is a remarkably increasing demand for wireless LAN systems as they achieve higher speed and become available at reduced costs. Particularly, introduction of a personal area network (PAN) is especially being considered to construct a small-scale network for information communication between electronic devices available around users. For example, there are provided different wireless communication systems and wireless communication apparatuses using frequency bands such as 2.4 GHz and 5 GHz bands that need not be licensed by governing legal authorities.
Typical wireless network standards include IEEE (The Institute of Electrical and Electronics Engineers) 802.11 (e.g., see non-patent document 1), HiperLAN/2 (e.g., see non-patent document 2 or 3), IEEE302.15.3, and Bluetooth communication. The IEEE802.11 standard further includes extension standards such as IEEE802.11a (e.g., see non-patent document 4), IEEE802.11b,. and IEEE802.11g depending on differences in the wireless communication systems and frequency bands to be used.
The IEEE802.11a standard supports modulation systems that reach up to 54 Mbps of communication speed. A need exists for wireless standards capable of higher bit rates in terms of communication speeds. In recent years, particular attention is placed on the MIMO (Multi-Input Multi-Output) communication technology. According to this technology, plural antenna elements are provided for both a transmitter and a receiver. The technology implements space division multiplexing, i.e., plural logically independent transmission paths (hereafter also referred to as “MIMO channels”) to enhance the transmission capacity and improve the communication speed. The MIMO communication uses the space division multiplexing to provide efficient use of frequencies.
FIG. 7 schematically shows a MIMO communication system. As shown in FIG. 7, a transmitter and a receiver each have plural antennas. The transmitter space/time-encodes plural transmission data, multiplexes and distributes it to M antennas for transmission to plural MIMO channels. The receiver receives a signal at N antennas via channels and space/time-decodes the received signal to obtain reception data. The MIMO communication differs from a simple transmission/reception adaptive array. In this case, a channel model comprises a transmitter-related radio wave environment (transfer function), a channel space structure (transfer function), and a receiver-related radio wave environment (transfer function). A crosstalk occurs during multiplexing of a signal transmitted from each antenna. The receiver can perform a signal process to correctly retrieve multiplexed signals without crosstalk.
In short, the MIMO communication system uses the transmitter to send transmission data by distributing it to plural antennas. The transmitter uses plural virtual MIMO channels for transmission. The receiver receives a signal via plural antennas and performs a signal process to obtain reception data. In this manner, the communication system uses channel characteristics. There are various systems as MIMO transmission construction methods. One of ideal modes is known as the SVD-MIMO system (e.g., see non-patent document 5) using singular value decomposition (SVD) of transfer functions.
FIG. 8 schematically shows an SVD-MIMO system. The SVD-MIMO transmission system finds UDVH by performing a singular value decomposition for a numeric matrix composed of channel information corresponding to each antenna pair, i.e., channel information matrix H. The system gives V as an antenna weight coefficient matrix for the transmitter and UH as an antenna weight coefficient matrix for the transmitter. Consequently, each MIMO channel is represented as diagonal matrix D whose diagonal element is a square root of unique value λi. Signals can be multiplexed for transmission with no crosstalk.
The following needs to be considered. It is not easy to realtime perform singular value decompositions. A need exists for a setup procedure to previously notify a destination of derived V or UH.
The SVD-MIMO transmission system can provide the theoretically maximum communication capacity. When the transmitter and the receiver each have two antennas, for example, the transmission capacity can be doubled maximumly.
The following describes a mechanism of the SVD-MIMO transmission system in detail. When the number of transmitter antennas is assumed to be M, transmitted signal x is represented as vector M×1. When the number of receiver antennas is assumed to be N, received signal y is represented as vector N×1. In this case, the channel characteristic is represented as numeric matrix H of N×M. Element hij of channel information matrix H is a transfer function from the jth transmission antenna to the ith reception antenna. As expressed in equation (1) below, the received signal vector y is represented by multiplying the transmitted signal vector by the channel information matrix and by adding noise vector n.y=Hx+n  (1)
As mentioned above, a singular value decomposition is performed for the channel information matrix H to result in equation (2) below.H=UDVH  (2)
where transmitter's antenna weight coefficient matrix V and receiver's antenna weight matrix U are unitary matrices satisfying equations (3) and (4) below, respectively.UHU=I  (3)VHV=I  (4)
That is, arranging unique vectors with normalized HHH constitutes receiver's antenna weight matrix UH. Arranging unique vectors with normalized HHH constitutes transmitter's antenna weight matrix V. D is a diagonal matrix and contains a diagonal element equivalent to a square root of unique value HHH or HHH. The size is the number of transmission antennas M or the number of reception antennas N whichever is smaller. The size is a square matrix of size min[M,N] and corresponds to a diagonal matrix.
                    D        =                  [                                                                                          λ                    1                                                                              ⋯                                                                                                                          0                                                                    ⋮                                                                                  λ                    2                                                                                                                                                                                                                                                                                                                                                                                                                              ⋰                                                                                                                                                  0                                                                                                                                                                                                                                              λ                                          min                      ⁡                                              (                                                  M                          ,                          N                                                )                                                                                                                          ]                                    (        5        )            
While there has been described the singular value decomposition for real numbers, some considerations are needed when the singular value decomposition is extended to imaginary numbers. It is assumed that U and V are matrices composed of unique vectors. Even though unique vectors are normalized to set the norm to 1, U and V do not become unitary, allowing the existence of many unique vectors with different phases. The above-mentioned equation (2) may be invalid depending on phase relationship between U and V. Even though U and V are correct, only the phases rotate independently. To enable phases to completely correspond to each other, V is normally found as a unique vector for HHH. As shown in the following equation, U is found by multiplying V and both terms of the above-mentioned equation (2) together from the right.HV=UDVHV=UDI=UDU=HVD−1  (6)
The transmitter provides weight using the antenna weight coefficient matrix V. The receiver provides weight using the antenna weight coefficient matrix UH for reception. Since U and V are unitary matrices (U is N×min[M,N] and V is M×min[M,N]), the following equation takes effect.
                                                        y              =                            ⁢                                                                    U                    H                                    ⁢                  HVx                                +                                                      U                    H                                    ⁢                  n                                                                                                        =                            ⁢                                                                                          U                      H                                        ⁡                                          (                                              UDV                        H                                            )                                                        ⁢                  Vx                                +                                                      U                    H                                    ⁢                  n                                                                                                        =                            ⁢                                                                    (                                                                  U                        H                                            ⁢                      U                                        )                                    ⁢                                      D                    ⁡                                          (                                                                        V                          H                                                ⁢                        V                                            )                                                        ⁢                  x                                +                                                      U                    H                                    ⁢                  n                                                                                                        =                            ⁢                              IDIx                +                                                      U                    H                                    ⁢                  n                                                                                                        y              =                            ⁢                              Dx                +                                                      U                    H                                    ⁢                  n                                                                                        (        7        )            
The received signal y and the transmitted signal x are (min[M,N]×1) vectors, not vectors determined by the number of transmission antennas and reception antennas.
Since D is a diagonal matrix, transmitted signals can be received without crosstalk. Since the amplitude of each independent MIMO channel is proportional to a square root of the unique value λ, the electric power magnitude for each MIMO channel is proportional to λ.
Since noise element n is also a unique vector having the U column whose norm is normalized to 1, UHn does not change the noise power. The size is the same as y and x because UHn is a (min[M,N]) vector.
Despite the same frequency and the same time, the SVD-MIMO transmission can provide plural logically independent MIMO channels without crosstalk. That is, this makes it possible to transmit plural pieces of data through wireless communication at the same time using the same frequency, improving the transmission speed.
Generally, the number of MIMO channels available for the SVD-MIMO communication system is equivalent to the number of transmission antennas M or the number of reception antennas N whichever is smaller, i.e., min[M,N]. The transmitter's antenna weight coefficient matrix V comprises transmission vectors vi for the number of MIMO channels (V=[v1, v2, . . . , vmin[M,N]). The number of elements for each transmission vector vi corresponds to the number of transmission antennas M.
[Non-patent document 1] International Standard ISO/IEC 8802-11:1999(E) ANSI/IEEE Std 802.11, 1999 Edition, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications
[Non-patent document 2] ETSI Standard ETSI TS 101 761-1 V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part 1: Basic Data Transport Functions
[Non-patent document 3] ETSI TS 101 761-2 V1.3.1 Broadband Radio Access Networks(BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part 2: Radio Link Control (RLC) sublayer
[Non-patent document 4] Supplement to IEEE Standard for Information technology-Telecommunications and information exchange between systems-Local and metropolitan area networks-Specific requirements-Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: High-speed Physical Layer in the 5 GHZ Band
[Non-patent document 5]
http://radio3.ee.uec.ac.jp/MIMO(IEICE_TS).pdf (as of Oct. 24, 2003)
According to the SVD-MIMO system, the receiver needs to obtain channel information matrix H, perform a singular value decomposition for H, and transfer VH out of the decomposed UDVH to the transmitter. Since the transmitter actually uses only V, the receiver needs to transfer V to the transmitter.
The following describes the information amount of the transmitter's antenna coefficient matrix V using an example of the 5 GHz pair OFDM (Orthogonal Frequency Division Multiplexing) communication system. It complies with IEEE802.11a, one of LAN systems to which the SVD-MIMO transmission is applied.
When it is assumed that there are three antenna elements for each of the transmitter and the receiver, there are 3×3 matrices for the transmitter's antenna coefficient matrices V. Therefore, nine elements are available. It is assumed that one element is represented by a real number and a complex number with the 10-bit accuracy and is needed for 52 carriers. Then, the receiver must feed 9360 bits back to the transmitter, i.e., 9360 bits=9 (the number of matrices)×2 (real part and imaginary part of the complex number)×10 bits×52 (the number of OFDM subcarriers).
The MIMO requiring the feedback is called a closed loop MIMO (contrary to an open loop MIMO). Before initiating communication, a closed loop SVD-MIMO system must feed information as much as 9360 bits back to the transmitter. Let us assume that an attempt is made for feedback using the BPSK (Binary Phase Shift Keying) and the OFDM according to the modulation at a half code rate. The BPSK is the most reliable modulation provided by IEEE802.11a. In this case, one OFDM can transmit only 24 bits. The time required is equivalent to 3900 OFDM symbols. This is not realistic.
The communication can start when the receiver first transfers the antenna weight coefficient matrix V to the transmitter. However, a chronological change also changes elements (i.e., transfer functions) of the channel matrix H. Chronological changes in the channel matrix H chiefly result from indoor changes of reflection paths and temperature due to the movement of human beings and objects.
When the channel characteristics chronologically change in this manner, the receiver needs to obtain the channel matrix H again and performs a singular value decomposition for the new H. The receiver feeds a new transmitter's weight coefficient matrix V back to the transmitter and requests transmission weighed by the new V (i.e., restudy of V). Further, the receiver itself needs to decode received signals using new UH.
During the SVD-MIMO transmission, the inventors consider it necessary to compress the amount of information to be fed back to the transmitter from the receiver.